This invention relates to aqueous polyisocyanate emulsions useful as binders in the production of molded articles, particularly boards of materials containing lignocellulose.
Molded articles such as chipboard, composite board and the like are generally produced by hot molding an inorganic or organic raw material (e.g. a mass of wood shavings, wood fibers or other material containing lignocellulose) together with binders (e.g. aqueous dispersions or solutions of urea formaldehyde or phenol formaldehyde resins). It is known to use isocyanate solutions instead of urea formaldehyde or phenol formaldehyde resins as binders for molded board (German Auslegeschrift No. 1,271,984, German Offenlegungsschrift No. 1,653,177).
In these hot molding processes (particularly in the manufacture of wood chipboards) it is generally necessary under the standard regulations of the building trade (e.g. DIN No. 68 763) to add hydrophobicizing agents separately. In practice, such agents are usually in the form of aqueous paraffin dispersions because it is believed to be impossible to mix these agents with unmodified polyisocyanates (Sachs, 1977, Holz-Zentralblatt No. 20, page 295/296; Ball/Redman, 1979, Holz-Zentralblatt No. 88, page 1291). Use of isocyanates for the manufacture of chipboard is thereby rendered much more difficult.
Another known process for the manufacture of boards or molded articles is hot molding a mixture of lignocellulose material and an aqueous emulsion of an organic polyisocyanates as binder (German Offenlegungsschriften Nos. 2,610,552 and 2,703,271). In this process, the aqueous paraffin dispersions required for the manufacture of wood chipboard may be applied separately or in admixture with a previously prepared aqueous isocyanate emulsion (Ball/Redman 1979, Holz-Zentralblatt No. 88, page 1291; German Offenlegungsschrift 2,948,015). However, these aqueous emulsions have the serious disadvantage that expensive isocyanate modification processes are required to render the emulsion hydrophilic and emulsifiable. In some cases the necessary emulsifiers are expensive and they increase the cost of the binders by 20 to 30% making such isocyanate binders less competitive economically with conventional binders. Additionally, isocyanates which have been modified to render them emulsifiable (see German Auslegeschriften Nos. 2,921,681; 2,921,726, 2,610,552 and 2,703,271 and the literature cited therein) give rise to storage problems of both the products themselves and the glue-covered chips.